1. Field of the Invention
The present invention relates to wideband homodyne receivers for use in communications and electronic warfare receiving system applications. In particular, the present invention is a novel homodyne receiver that reduces spurious outputs.
2. Description of the Related Art
Modem communication, radar, and related receiving systems often include a means for converting an input signal at a frequency .function..sub.R to another, often lower, frequency .function..sub.I to facilitate detection, demodulation and processing of the received signals. A number of receiver architectures have been developed over the last half decade for this purpose including superheterodyne, instantaneous frequency measurement and homodyne receivers. A superheterodyne receiver converts an input signal centered at a frequency .function..sub.R to a replica of the input signal centered at a second frequency .function..sub.I by mixing the input signal with a variable frequency local oscillator (LO) signal. The frequency .function..sub.I is often referred to as the intermediate frequency (IF). Hereinafter, the term mixing refers to the time domain multiplication of a pail of signals in a mixer having a pair of input ports or terminals referred to as the RF and LO ports and a single output port called the IF port.
A homodyne receiver is a specialized superheterodyne receiver in which the local oscillator (LO) is derived from a combination of the input signal and a fixed frequency offset oscillator rather than from an independent variable frequency oscillator. A homodyne receiver converts all input signals within its input bandwidth to a single IF output frequency band without the need for a variable frequency local oscillator. Homodyne receivers are potentially attractive receivers in many applications where the cost associated with a complex, variable frequency local oscillator is prohibitive or where there is some uncertainty about the exact frequency of the input signal.
FIG. 1 illustrates a block diagram of a basic homodyne receiver. The basic homodyne receiver includes a coupler 10, a limiting amplifier 12, a first mixer 14, a fixed frequency offset oscillator 16, a second mixer 18 and an intermediate frequency (IF) bandpass filter 20. A limiting amplifier herein is an amplifier that accepts an input signal with a potentially varying amplitude and produces a constant amplitude signal at the same frequency as the input signal. The offset oscillator 16 produces a single, sinusoid at frequency .function..sub.L. The RF and LO ports of the mixers 14, 18 are designated R and L respectively and the IF port is designated I. A signal entering the basic homodyne receiver is mixed with a frequency offset version of itself to produce a signal at the desired IF frequency. The IF frequency .function..sub.I to which the input signal is thereby converted equals the frequency of the offset oscillator 16, namely .function..sub.L.
It is well known in the art that an arbitrary input signal can be decomposed into a linear combination of individual frequency or spectral components. The combination of these frequency components is referred to as the spectrum of a signal. The use of an input signal with a single frequency component .function..sub.R is not intended to limit the applicability of the discussion that follows.
The operation of the basic homodyne receiver of FIG. 1 can be understood by considering an input signal centered at a frequency .function..sub.R. A sample of the input signal .function..sub.R, after being amplified to a constant level by the limiting amplifier 12, is mixed with the output of the offset oscillator 16. The first mixer 14 then generates an output that includes, among other spectral components, a component that is the sum (.function..sub.R +.function..sub.L) of the frequencies at its RF and LO ports. The signal designated .function..sub.R hereinafter refers to the input signal after it has passed through the limiting amplifier 12 to distinguish it from the original input signal .function..sub.R. Both .function..sub.R and .function..sub.R are understood to have the same center frequency.
The output of the first mixer containing the spectral component (.function..sub.R +.function..sub.L) is then mixed with the original input signal .function..sub.R in the second mixer 18. The output of the second mixer 18 includes the desired spectral component given by equation (1). EQU (.function..sub.R +.function..sub.L)-.function..sub.R =.function..sub.L =.function..sub.I,d ( 1)
According to equation (1), the output of the homodyne receiver has a component at .function..sub.I,d =.function..sub.L regardless of the input signal frequency .function..sub.R.
Now consider an input signal centered at .function..sub.R with amplitude and/or phase modulation. With this input, the homodyne receiver produces an output signal component centered at .function..sub.L and having amplitude and/or phase modulation that is proportional to that of the input signal. Moreover, the IF output center frequency will not change if the center frequency .function..sub.R of the input signal changes. The output center frequency .function..sub.I is determined by the fixed offset oscillator 16 frequency .function..sub.L and not by the input signal center frequency allowing the homodyne receiver to instantaneously track frequency changes in the input signal.
As alluded to above, the desired frequency conversion product .function..sub.I,d of equation (1) is only one of many products that results from passing an input signal through the homodyne receiver. Real mixers used in receivers generate a large number of spectral components at their outputs in addition to the desired component. The spectral components generated by a real mixer represent linear combinations of the signals present at the LO and RF ports of the mixer and are given by equation (2) EQU .function..sub.I =M.multidot..function..sub.R +N.multidot..function..sub.L( 2)
where M and N are integers. The spectral components described by equation (2) are known to vary in amplitude relative to the input signal amplitudes. Generally, the amplitudes of the spectral components decrease as the magnitudes of M and/or N increase for real mixers. This decrease means that only the smaller values of M and N are of interest in most designs using mixers. Generally, magnitudes of M and N less than 2 or 3 are the only spurious components with high enough power to be of concern. Since only one pair of {M, N} values represents a desired product in a given mixing application, undesired spectral components will always be present when mixing is employed and must be considered in any application using mixers.
As a minimum, equation (2) indicates that the multiplication of a pair of sinusoids with frequencies .function..sub.R and .function..sub.L, always results in a pair of sinusoids with frequencies (.function..sub.R +.function..sub.L) and (.function..sub.R -.function..sub.L) normally referred to as the upper and lower sidebands respectively. The components that represent undesired conversion products and designated hereinafter as .function..sub.I,s are referred to as spurious responses or simply spurs. The ratio of the spurious output power to the desired component is referred to as the spurious level. The spurious rejection capability of a device is a figure of merit obtained by subtracting the spurious level from one.
Returning now to the case of the first mixer 14 in the homodyne receiver of FIG. 1. The desired signal is (.function..sub.R +.function..sub.L). Other components present at the output of mixer 14 include .function..sub.R, .function..sub.L, 2.function..sub.R, 2.function..sub.L, (.function..sub.R -.function..sub.L), (2.function..function..sub.R +.function..sub.L) as predicted by equation (2). Some of these components can be removed by using a filter between the first mixer 14 and the second mixer 18 but many cannot. Filtering to remove spurious components is especially difficult if the homodyne receiver has a wide input operational frequency range.
In the second mixer 18, the desired component is the lower sideband component represented by the difference between the frequencies at the RF and LO ports. The second mixer 18 will also generate a large number of spurious components. In addition, spurious components from the first mixer 14 are present to add to the number of possible undesired output spurs from the second mixer 18.
FIG. 2 shows an improved homodyne receiver known in the art that overcomes some of the spurious related problems of the simple homodyne receiver of FIG. 1. As with the basic homodyne receiver, the improved homodyne receiver of FIG. 2 has a coupler 10, a limiting amplifier 12, a local oscillator 16, and an IF filter 20. The improved homodyne receiver, however uses an upper sideband single sideband modulator 22 in place of the first mixer 14 and an image reject mixer 24 instead of the second mixer 18. In addition, there is a highpass filter 26 between the single sideband modulator 22 and the image reject mixer 24.
The single sideband (SSB) modulator 22 is a device or circuit, usually including a plurality of mixers, that generates a single sideband, suppressed carrier, modulated output signal from an applied modulating waveform and an un-modulated carrier signal. The SSB modulator has a pair of input ports often referred to as RF or R and LO or L and one or two output ports often called IF ports. Modulator 22 mixes the signal present at the inputs to produce an output signal. The ideal SSB modulator differs from a mixer in that the output signal contains only one of the two sidebands associated with the mixing process. In SSB modulators with two output ports, one generally provides the upper sideband (.function..sub.R +.function..sub.L) while the other provides the lower sideband (.function..sub.R -.function..sub.L). SSB modulators with single output ports are internally configured to produce either the upper or lower sideband product and are specified as being either upper sideband SSB modulators or lower sideband SSB modulators.
A block diagram of a single sideband (SSB) modulator known in the art is illustrated in FIG. 3. The SSB modulator of FIG. 3 consists of an input 90-degree hybrid coupler 30 and a pair of mixers 32, 34, an output 180-degree hybrid coupler 36 and a 90-degree hybrid 38. The mixers 32, 34 are preferably double balanced mixers. The desired sideband, either upper sideband or lower sideband, of the modulated carrier can be selected by choosing the proper port 45 or 46 of the 180-degree hybrid 36 and terminating the unused port with a matched load.
In real SSB modulators, suppression of one sideband relative to the other at a given output port of the SSB modulator is not perfect. This is due to non-ideal characteristics of the hybrids and mixers that are used in the circuit. Typical SSB modulators are known in the art to achieve approximately 15 to 20 dB suppression of the unwanted sideband over multi-octave input bandwidths. As with mixers, the output of typical SSB modulators will also contain components at the frequency of the input signals and at other spurious frequencies given by equation (2).
The image reject mixer 24 of the improved homodyne receiver of FIG. 2 is a specialized mixer design that suppresses the mixer's input image response. An image reject mixer generally has a pair of input terminals called RF or R and LO or L and a pair of output terminals. As with the SSB modulator, the image reject mixer 24 is not ideal and only suppresses but does not completely remove the unwanted input sideband.
A block diagram of an image reject mixer known in the art is illustrated in FIG. 4 and has an input port 50, a local oscillator port 51, an upper sideband IF port 52, and a lower sideband port 53. The illustrated image reject mixer includes a pair of 90-degree hybrids 54, 55. A pair of preferably double balanced mixers 56, 57 and a power divider 58. The action of the image reject mixer is to preferentially suppress one of the input sidebands at each of the two output ports 52, 53. Selection of the desired input sideband is accomplished by choosing the appropriate port of the output hybrid 55 and terminating the unused port. The improved homodyne receiver of FIG. 2 requires the image reject mixer 24 to provide the (.function..sub.R -.function..sub.L) or lower sideband, while the suppressing the (.function..sub.R +.function..sub.L) or upper sideband.
In the improved homodyne receiver of FIG. 2, the use of the SSB modulator 22 such as the one illustrated in FIG. 3 and the image reject mixer 24 such as that of FIG. 4 lowers the spurious content of the IF output of the improved homodyne receiver of FIG. 2 as compared to that of the basic homodyne receiver of FIG. 1. The output of improved homodyne receiver has three principle spectral components given by equations (3), (4) and (5) below that are of concern to most practical applications of homodyne receivers. EQU (.function..sub.R +.function..sub.L)-.function..sub.R =.function..sub.L =.function..sub.I,d ( 3) EQU .function..sub.R -(.function..sub.R -.function..sub.L)=.function..sub.L =.function..sub.I,s1 ( 4 ) EQU (.function..sub.R +.function..sub.L)-.function..sub.R =.function..sub.L =.function..sub.i,s2 ( 5)
Equation (3) represents the desired IF output component while equations (4) and (5) represent spurious responses. The spurious spectral components of equations (4) and (5) are of critical concern because these components are centered at the same frequency, namely .function..sub.I, as the desired IF component of equation (3).
The frequency component given by equation (4) is the LO image spur created by the mixing of the input signal at .function..sub.R with the suppressed lower sideband in the SSB modulator 22. Since the SSB modulator 22 suppresses the lower sideband (.function..sub.R -.function..sub.L) by about 20 dB, this spurious component will be approximately 20 dB lower than the desired component .function..sub.I,d. The spurious response of equation (5) is the result of the self mixing of the two signals present at the LO port of the image reject mixer 24. Self mixing at the LO port is analogous to and is generated by the same mechanism as the 2.function..sub.L spur of equation (2) except self-mixing occurs when two signals are present simultaneously at the LO or RF port. Since the SSB modulator 22 does not completely suppress the .function..sub.R component present at its input, this component can mix with the (.function..sub.R +.function..sub.L) component to produce an output at the IF frequency. The .function..sub.R component present at the LO port of the image reject mixer 24, unlike the input signal, is constant in amplitude due to the action of the limiting amplifier 12. Likewise, the spurious IF signal centered at .function..sub.I,s2 produced by the mixing product of equation (5) will be constant in amplitude. Even though the .function..sub.R component in the output of the SSB modulator 22 is suppressed, the IF spurious component .function..sub.I,s2 could be large compared to desired component .function..sub.I,d for small input signals.
The spurious components present in the IF signal can cause severe problems in applications where a homodyne receiver is used to generate an IF output signal with phase and amplitude variations or modulations that are proportional to the phase and amplitude variations or modulation of the original input signal. Spurs present in the IF signal will introduce measurement errors when the phase and/or amplitude is measured. The larger the spurious levels are relative to the desired signal level, the larger are the errors that are introduced. The measurement of phase modulation is particularly susceptible to errors caused by the spurious signal components. The phase error introduced by a single spurious component 20 dB below the desired signal component is about 5.5 degrees. In many applications, phase errors of more than a degree or two are considered unacceptable.
Therefore, it would be desirable to have a homodyne receiver that has lower spurious component levels or better spurious rejection characteristics than are possible with conventional homodyne receivers known in the art. Such an improvement in the homodyne receiver would overcome a long standing problem in the area of signal receiver technology.